 //回到顶部
    function arrow(){
        var tt = null;
        tt = requestAnimationFrame(function arro(){
            var oTop = document.body.scrollTop || document.documentElement.scrollTop;
            if(oTop>0){
                document.body.scrollTop=document.documentElement.scrollTop = oTop-30;
                tt = requestAnimationFrame(arro);
            }else{
                cancelAnimationFrame(tt);
            }
        });
    }

